1. Field of the Invention
This invention relates to a photosensitive lithographic plate (hereinafter referred to as "PS plate"), and more particularly to a PS plate of negative type photopolymer.
2. Description of the Prior Art
Diazo type photosensitive materials are often used for photosensitive layers of PS plates. Examples of known diazo type photosensitive materials include, o-naphthoquinone diazide used in positive type PS plates and a condensate of 4-diazo-diphenylamine, 3-methoxy-4-diazo-diphenylamine, or 2,5-diethoxy-4-diazo(tolylmercapto)-benzene with paraformaldehyde used in negative type PS plates.
By using a mixture of a diazo resin and another high molecular weight compound, a lithographic printing plate can be produced that can withstand the impacts of printing without requiring any lacquer padding and can be easily developed with an aqueous developing liquid that contains a surface active agent. Diazo resins are used in large amounts for photosensitive layers of negative type PS plates. The diazo resin used in the production of such PS plates is in a water-insoluble form, as disclosed in the specification of Japanese Patent Application Publication No. 47(1972)-1167 to provide enhanced storage stability of the PS plates. The diazo resin, however, is vulnerable to heat and moisture when prepared in the form of a solution. For this reason, negative type PS plates using a diazo resin are inferior to PS plates using solvent-developing type photopolymers with respect to properties such as the stability to withstand aging due to production lot, the strength of formed image, and the resistance to impacts of printing.
When the diazo resin is mixed with another high molecular weight compound, the resultant photosensitizing solution is unstable at room temperature and must be used within several hours to several days after the diazo resin and the high molecular weight compound are mixed. Further, when the diazo photosensitizing solution is mechanically kneaded as with a roller coater, the decomposition of the diazo compound is accelerated so that the photosensitizing solution becomes unusable in several hours.
Thus, from the moment the diazo resin is dissolved in the solvent, its decomposition steadily proceeds with elapse of time. The diazo photosensitizing solution having a high molecular weight compound mixed therein is very difficult to preserve. The production of PS plates using such diazo photosensitizing solutions is extremely difficult in the summer when temperature and humidity are high.